Thermionic emission. Phenomenon that has its origin in the emission of electrical particles with negative charge or electrons from an electrically heated filament in all directions.
Summary
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- 1 History
- 2 Edison experiment
- 3 Richardson’s Law
- 4 Sources
History
Initially reported in 1873 by Frederick Guthrie in Britain. While conducting experiments with charged objects, Professor Bernd Schuster discovered that by heating a negatively charged iron sphere red hot, it lost its charge (giving off ions in a vacuum). He noted that this did not occur if the sphere was positively charged. Other investigations in this regard included Hittorf ( 1869 – 1883 ), Goldstein (1885) and Elster and Geitel (1882-1889)
The effect was rediscovered by Thomas Edison on 13 of February of 1880 , while trying to discover the reason why the filaments were broken and why the glass is dark (the bulb were smoked near one of the terminals of the filament) of its incandescent lamps .
Edison was successful with it and throughout the years he was perfecting it to improve its durability, the cost of manufacture, the use of light and the preservation of its qualities over time. Regarding the latter, Edison realized that although the incandescent filament remained relatively intact until the moment of collapse, the glass of the bulb was gradually darkening.
He assumed that “something” would detach from the filament, try to escape from the bulb, and get stuck against the glass. Since Edison was more of an inventor than a scientist, he did not spend much time explaining the phenomenon but instead concentrated on solving it.
What Edison did to correct the phenomenon was to introduce a metal plate inside the bulb which was polarized with a positive charge . With this, he observed that the darkening of the glass was minimized since that “something” that detached from the filament was going to hit the loaded plate directly. He then went to his friends at the patent office and registered the system as the Edison Effect, without stopping to find out how it could be used and also without thinking that in 1873 the English scientist Frederick Guthrie had already described this phenomenon as Thermoionic Emission.
Edison experiment
Flow of charged particles called ions that comes from a metal surface (or metal oxide) caused by a vibrational-type thermal energy that causes an electrostatic force that pushes electrons to the surface. The charge on the thermions (which can be positive or negative) will be the same as the charge on the metal or metal oxide. The effect increases dramatically with increasing temperature (1000–3000 K). The science that studies this phenomenon is thermionics.
The experiment that Edison carried out to see how his new improvement worked was as follows:
When the plate had a more positive charge than the filament, the electrons flowed (Electron flow), while when the charge on the plate was more negative than that of the filament, they did not flow (No current).
The electrons do not advance against the transit and that now we all know. At the time, instead, it meant something magnificent: the ability to have a switch – letting current flow through or interrupting it – without requiring an operator to move a lever or press a button. This simple device made it possible to activate or stop the flow without any intervention other than the variation in the load of the components.
In this experiment, Edison also discovered that the current emitted by the hot filament increased rapidly with increasing voltage and presented an application for a voltage regulating device using this effect on November 15, 1883 (US Patent 307,031 – the first patent of an electronic device). He proposed that enough current could pass through the apparatus to operate a sound telegraph.
With the above, the arrival of the vacuum tube was imminent and electronics began to develop worldwide thanks to this simple invention, which led to the creation of different tubes around the precursor developed by Edison. As in current electronics, everything is based on the ability to generate ones and zeros, which results in more complex combinations that allow AND and OR gates, among others. Edison unknowingly contributed to electronics the generator of ones and zeros.
Exhibited at the International Exposition of Electicity in Philadelphia in September 1884. William Preece, a British scientist took some of the Edison Effect bulbs with him, and presented a study on them in 1885, where he referred to thermionic emission as the “Edison effect.” British physicist John Ambrose Fleming, working for the British company “Wireless Telegraphy”, discovered that the Edison Effect could be used to detect radio waves. Fleming began developing the 2-element vacuum tube, known as a diode, which he patented on November 16, 1904.
The thermionic diode can also be configured as a device that converts a temperature difference into electrical energy directly without moving parts (a thermionic converter, a type of heat generator).
Richardson’s Law
In any metal, there are one or two electrons per atom that are free to move from one atom to another. This is called a “sea of electrons.” Its speed, rather than being uniform, is modeled by a statistical distribution, and occasionally an electron will have enough speed to escape from the metal, without being drawn back. The minimum amount of energy required for an electron to escape from the surface is called the work function. This work function is characteristic of the material and for most metals it is on the order of several electron volts. Thermionic currents can be increased or decreased by decreasing the work function. This feature, which is highly desirable, can be achieved by applying various oxide coatings to the wire.
In 1901 , Owen Willans Richardson published the results of his experiments: the current from a wire, under controlled heating, seemed to depend exponentially on the temperature of the wire, a behavior that was modeled by a mathematical formula similar to the Arrhenius equation. The modern form of this law (demonstrated by Saul Dushman in 1923 , and therefore sometimes called the Richardson-Dushman equation) states that the emitted current density is related to the temperature T by the equation:
where T is the metal temperature in kelvin, W is the metal work function, k is Boltzmann’s constant. The constant of proportionality A, known as Richardson’s constant, given by,
where m and -e are the mass and charge of the electron, and h is Planck’s constant.
